It is intended to provide a polyethylene powder which can offer a fiber excellent in resistance to end breakage, dimensional stability, and acid resistance and/or a microporous membrane excellent in dimensional stability and acid resistance, and a microporous membrane and a fiber which are obtained by forming the polyethylene powder. The present invention provides a polyethylene powder comprising: 0.5 ppm or higher and 3,000 ppm or lower of aluminum hydroxide having an average particle size smaller than 50 μm; and 0.5 ppm or higher and 12 ppm or lower of a magnesium element, wherein the polyethylene has a viscosity-average molecular weight of 100,000 or larger.